1. Field of the Invention
This invention relates to fiber optic devices. More particularly, this invention relates to the integration of fiber optic coupling devices with optical isolators. Further, this invention relates to ultra stable fiber optical devices utilized in coupling arrangements integrated with isolators, for instance in fiber tap applications and fiber optic amplification applications.
2. The Prior Art
Fiber optical couplers and isolators exist in the prior art. Each of them performs a specific function in the fiber optical system. For example, one type of fiber optical coupler can be used to split optical power into multiple channels with various splitting ratios. Another type of fiber optical coupler can perform the function of wavelength division multiplexing and demultiplexing. Optical isolators have been used to allow light to propagate only in one direction, the light propagating in the other direction will be directed into a different optical path and suffers a significant loss in such prior devices. In an optical system where both a coupler and an isolator are needed, they are usually combined with a fusion splice (such as those depicted in U.S. Pat. No. 5,375,010). This type of integration is usually bulky and not cost effective. In addition, a splice causes loss, which adds to the overall loss of the optical system. Thus, there is a need to produce smaller, less expensive and low loss integrated coupler-isolator devices. Specifically, an integrated tap-isolator and WDM-isolator is of great interest to optical system developers.
Taps for optical fibers are known in the prior art. The purpose of such taps is to bleed off a small portion of optical signal to analyze the signal for desirable characteristics. One example of an integrated tap-isolator or WDM-isolator as exists in the prior art that uses a dielectric thin film reflector is shown in FIG. 1. However, such prior art taps suffer from a number of shortcomings. First, the tap ratio is highly dependent on the characteristics of the dielectric thin film filter which is sometimes sensitive to environmental variation such as temperature and humidity. A temperature insensitive filter is thus required in most applications, which adds cost. Further, the tapping is not provided directly on the output of the device or on the input of the device. In other words, the optical coupling of the tap port to the input port is different to that of the output port to the input port. Environmental variations such as temperature, humidity and stress will affect the optical coupling differently with respect to the tap port and the output port. The variation of optical coupling of tap port and output port will create variations in the tap ratio with respect to environmental changes which is not desirable in the optical systems.
In addition, such prior art taps also suffer from various other shortcomings. For example, many such prior art taps rely on a reflective tap coating. However, it is known that the optical characteristics of such coatings have a tendency to change with temperature as well and this may adversely affect the tap performance. Furthermore, many prior art taps depend on mechanical alignment to perform as desired. As one can appreciate, and as is well known in the prior art, precise alignment of prior art taps is difficult at best. Failure to properly align the tap or mechanical shock, e.g., from dropping the unit, will result in additional unwanted optical losses.
U.S. Pat. No. 5,657,155 to Cheng attempts to overcome the traditional tap problem by employing a holographic beam splitter (HBS) instead of a reflector/splitter in various fashions with and without wavelength division multiplexers (WDMs). However, the addition of an HBS to the tap circuit creates its own problems and complications, not the least of which is cost. The HLBS is usually costly and lossy. In addition, it relies on optical alignment of the output port and the tap port to split input optical beams at the same time. Manufacturing of this type of device is non-trival and it suffers the same problem of variation of tap ratio with respect to environmental changes as explained in the case of dialectic thin film tap-isolators.
To overcome these and other shortcomings of the prior art, disclosed herein is an integrated isolator fused coupler. By utilizing improved fused biconical tapered couplers (tap or WDM types) in combination with a simplified isolator circuit, in various forms, such will not only result in a smaller device, but improved optical performance is also achieved.
The inventive integrated isolator fused couplers disclosed herein have many advantages over the prior art. For instance, by employing all fiber components for taping or WDM applications, the circuit is less sensitive to wavelength characteristics and temperature. Additionally, the inventive integrated isolator fused couplers are not dependent on mechanical alignment. As a tap, the invention taps the desirable output signal directly with fewer components than employed in prior art devices and with reduced loss. Furthermore, by packaging the fused fiber coupler into the isolator subassembly, i.e. collimator/ferrule, a reduced footprint is achieved.
It is therefore a primary object of the present invention to provide an integrated isolator fused coupler with less overall additional components as compared to the prior art.
It is another object of the present invention to provide an integrated isolator fused coupler embodied as a tap that includes both low loss and increased signal throughput.
It is another object of the present invention to provide an integrated isolator fused coupler in a forward pumping arrangement.
It is yet another object of the present invention to provide an integrated isolator fused coupler combining both a forward pumping arrangement and a tap at an output end thereof.
It is also an object of the present invention to provide an integrated isolator fused coupler that is smaller in size as compared to similar prior art devices, wherein tapping or WDM functions are performed utilizing a fused fiber approach, and is therefore not as sensitive to temperature, light characteristics, and humidity as compared to prior art devices.
Viewed from one vantage point, a fiber optic tap is disclosed, comprising in combination at least one fiber optic coupler and a fiber optic isolator optically coupled to said fiber optic coupler wherein said fiber optic coupler is integrated in a subassembly of said fiber optic isolator.
Viewed from another vantage point, a fiber optic WDM-isolator assembly is disclosed, comprising in combination a wavelength division multiplexer and a fiber optic isolator optically coupled to said wavelength division multiplexer.
Viewed from another vantage point, a fiber optic WDM-pump combination is disclosed, comprising in combination a first fiber optic coupler, a fiber optic isolator optically coupled to said first fiber optic coupler, and a second fiber optic coupler optically coupled with said fiber optic isolator.
Viewed from yet another vantage point, a fiber optic assembly is disclosed, comprising in combination an input fiber partially encased within a ferrule and extending from said ferrule, a first lens optically coupled to said input fiber, an optical isolator optically coupled to said first lens, a second lens optically coupled to said optical isolator, and a fiber optic coupler encased within a ferrule, said fiber optic coupler including a first pair of pigtail ends and a second pair of pigtail ends, one of said first pair of said pigtail ends optically coupled to said second lens.